A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 Hz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*106 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrowcasting systems or in cable-casting systems. The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) for terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data field each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S,xe2x88x92S,xe2x88x92S and +S. The value +S, is one level below the maximum posit ve data excursion, and the value xe2x88x92S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signa for channel-equalization and multipath suppression procedures. The training signal is a 511 sample pseudo-random noise sequence (or xe2x80x9cPN-sequencexe2x80x9d) followed by three 63-sample PN sequences. The middle one of these 63-sample PN sequences is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one""s complementary with respect to each other. The other two 63-sample PN sequences and the 511-sample PN sequence are transmitted in accordance with the same logic convention in all data fields.
The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded after having been randomized and subjected to diagonal byte interleaving. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a ⅔ rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction-coded (ECC) data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without preceding.
The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of xe2x88x927, xe2x88x925, xe2x88x923, xe2x88x921, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of+S is+5, and the normalized value ofxe2x88x92S isxe2x88x925.
The VSB modulation transmitted to DTV receivers in accordance with the September 1995 standard differs from that transmitted to NTSC receivers in that the low-end roll-off of the vestigial sideband ends in the portion of the frequency spectrum containing the lower frequencies of the more complete sideband, with the overall channel response being reduced 3 dB at frequencies close to the frequency of the suppressed carrier. Insofar as the high-end roll-off of the complete sideband is concerned the overall channel response of VSB modulation transmitted to DTV receivers is already reduced 3 dB at frequencies close to baud rate. DTV receiver designs proposed by the Grand Alliance further reduce the overall channel response at upper frequencies offset from carrier close to baud rate; this high-end roll-off introduces a Nyquist slope limitation on overall channel response, which is desired for minimizing intersymbol interference (ISI). The DTV receiver designs proposed by the Grand Alliance introduce further low-end roll-off beginning in the portion of the frequency spectrum containing the lower frequencies of the more complete sideband. This further low-end roll-off reduces the response at carrier frequency by another 3 dB at frequencies close to the frequency of the suppressed carrier, to match the Nyquist slope roll-off. When the DTV signals are demodulated, the amplitude response of the demodulation result is essentially flat at frequencies close to zero frequency, easing problems with equalizing at lower frequencies of baseband symbol response.
Beginning the low-end roll-off below carrier frequency introduces other problems, however. The resultant asymmetry of passband affects the operation of phase lock loops, as used for automatic fine tuning (AFT) of the first local oscillator in the DTV receiver, and as used for phase tracking of synchronous detection to recover baseband symbol code. The passband asymmetry exacerbates the problem of carrier jitter that occurs when the DTV signals that are being synchronously detected have poor carrier-to-noise ratio.
A better design compromise for the DTV system, the inventor observes, is to roll off the vestigial sideband without reducing amplitude response through the region immediately surrounding carrier frequency. The resulting boost in baseband amplitude response near zero frequency can be taken care of by equalization filtering, which is required anyway for overcoming multipath distortion.
A description of the characteristics of undesirable artifacts that arise in ATSC DTV signals owing to co-channel interfering NTSC analog TV signals is included in U.S. Pat. No. 5,835,131 issued Nov. 10, 1998 to A. L. R. Limberg, titled xe2x80x9cDIGITAL TELEVISION RECEIVER WITH ADAPTIVE FILTER CIRCUITRY FOR SUPPRESSING NTSC CO-CHANNEL INTERFERENCExe2x80x9d, and incorporated herein by reference. U. S. Pat. No. 5,835,131 also describes comb filtering methods that will suppress these artifacts.
The compromise with best noise performance of a DTV signal receiver that is required with the comb filtering approach to suppressing the artifacts of NTSC co-channel interference was recognized early on by the proponents of the DTV signal specified in the Digital Television Standard published by the ATSC Sep. 16, 1995. The fact of two independent paths for signals being combined in the comb filtering used for suppressing these artifacts increases noise energy while signa energy is decreased to result in a reduction of signal-to-noise ratio.
The transmission of VSB modulation with the full sideband located higher in frequency than the vestigial problem has been found to have a number of further problems in regard to transmission and reception, now that more experience has been gained with the proposed system. Co-channel interference of ATSC DTV signals with NTSC analog TV signals and co-channel interference of NTSC analog TV signals with ATSC DTV signals have been found to be more serious problems than originally contemplated. Co-channel interference of NTSC signals with ATSC DTV signals disrupts channel equalization. The use of comb filters for suppressing artifacts of NTSC co-channel interference and the attempt to achieve channel equalization tend to work in opposite directions.
The upper frequency limit of the VSB modulation is too close in frequency to the frequency-modulated audio carrier of an interfering co-channel NTSC signal. It is extremely difficult to suppress the FM audio signal with frequency-selective filtering in the IF amplifiers of DTV signal receivers. If the lower sidebands of the FM audio signal are suppressed as much as a receiver designer would like them to be, the phase response at the shoulder of the IF amplifier amplitude response is affected. Since the VSB signal is in effect single sideband at the shoulder, phase error markedly affects synchronous detection results for a signal component that is critical when baud rate close to channel capacity is sought.
The predictably high spatial correlation of video signal components of NTSC co-channel interference facilitates the use of comb filtering in a DTV signal receiver to suppress artifacts arising from these video signal components. However, the FM audio signal component of the NTSC signal exhibits little correlation over delay intervals of more than a few symbol epochs. If the FM audio carrier has modulated ultrasonic subcarriers, as is generally the case in recent times, predictability of correlation is poor even over a few symbol epochs. So, comb filtering is not a reliable way to reduce artifacts of NTSC FM sound carrier in a DTV signal receiver. The use of modulated ultrasonic subcarriers in the NTSC FM audio signal makes it impractical to use a tracking filter in a DTV signal receiver for suppressing that signal. A primary objective of the inventor was to mitigate the problems of co-channel interference from the FM audio signal component of the NTSC signal.
The inventive insight is that one can build a DTV signal receiver in which artifacts of the FM audio signal component of a co-channel NTSC signal do not appear, provided that the VSB modulation of the DTV signal (a) is not rolled off in the frequency region flanking carrier frequency and (b) is reversed so the DTV carrier is closer to the upper limit frequency of the channel than to its lower limit frequency. The vestigial sideband of the DTV signal is caused to overlap in frequency the FM audio signal component of a co-channel NTSC signal, and the full sideband of the DTV signal is located below the vestigial sideband in the frequency spectrum. An aspect of the invention is modifying the transmission of the DTV signal in this way.
Another aspect of the invention is a DTV signal receiver designed to receive these modified transmissions of DTV signal by synchronously detecting the DTV signal and then equalizing the resulting baseband DTV signal. Such a DTV signal receiver, because of the double-sideband (DSB) nature of the signal at lower modulation frequencies, reduces by 6 dB the artifacts generated by the audio signal of NTSC co-channel interference as compared to a similar receiver designed to receive the ATSC DTV signal.
Another aspect of the invention is embodied in a DTV signal receiver for receiving the modified transmissions of DTV signal, in which DTV signal receiver artifacts of the FM audio signal component of a co-channel NTSC signal do not appear. The DTV signal receiver includes a complex demodulator that recovers in-phase and quadrature-phase baseband signals. The in-phase baseband signal is composed of the regenerated DTV symbol coding, including low-frequency components thereof, and artifacts of co-channel interfering NTSC. The quadrature-phase baseband signal is composed of Hilbert transforms of the regenerated DTV symbol coding, excluding low-frequency components thereof, and artifacts of co-channel interfering NTSC. The quadrature-phase baseband signal is applied to an inverse-Hilbert-transform filter to recover the regenerated DTV symbol coding, excluding low-frequency components thereof, and artifacts of co-channel interfering NTSC. The components of this inverse-Hilbert-transform filter response are, except for the latency of the inverse-Hilbert-transform filter, the same as corresponding components of the in-phase baseband signal. The inverse-Hilbert-transform filter response is applied to a band separation filter that supplies a lowpass response and a highpass response. The lowpass response selects the artifacts of the audio signal component of co-channel interfering NTSC from the inverse-Hilbert-transform filter response, and the highband response selects the high-frequency components of regenerated DTV symbol coding from the inverse-Hilbert-transform filter response. The in-phase baseband signal is subjected to delay that compensates for the latencies of the inverse-Hilbert-transform filter and of the band separation filter. The delayed in-phase baseband signal has the lowpass response of the band separation filter combined therewith to cancel artifacts of the audio signal component of co-channel interfering NTSC and has the highpass response of the band separation filter combined therewith to boost the high-frequency components of the regenerated DTV symbol coding in the delayed in-phase baseband signal to the same gain level as the low-frequency components of the regenerated DTV symbol coding therein. The delayed in-phase baseband signal as so modified is a symbol coding stream with reasonably constant gain across baseband up to 5.5 MHz or so, which is supplied to channel equalization and symbol synchronizer circuitry. The response of the channel equalization and symbol synchronizer circuitry is supplied to a symbol decoder and is essentially free of artifacts of the audio signal component of co-channel interfering NTSC.
Another aspect of the invention is that a DTV signal receiver designed to receive these modified transmissions of DTV signal can use trap filtering for NTSC video carrier and lower-frequency modulation thereof in its intermediate-frequency amplifier circuitry without appreciably affecting the capability of the receiver for receiving DTV transmissions. This is facilitated because there is less crowding of critical frequencies of the DTV modulation at the lower-frequency end of the TV transmission channel when the carrier is placed near the upper limit frequency of that channel. There can also be trap filtering in the DTV signal receiver IF amplifier circuitry for NTSC chroma subcarrier and lower-frequency modulation thereof.
Another aspect of the invention is that, in a DTV signal receiver designed to receive the modified transmissions of DTV signal, choosing the DTV carrier frequency to be about 270 kHz below the upper limit frequency of the TV transmission channel facilitates the use of comb filtering to suppress NTSC chroma subcarrier and lower-frequency modulation thereof and to suppress NTSC video carrier and lower-frequency modulation thereof.